Some short propriospinal neurons are crucial for the production and coordination of vertebrate locomotion. Some of these must receive the descending and/or segmental input that initiates or regulates locomotion. We will work on the problems of determining the organization of short (1-5 segments) propriospinal neurons, and determining which populations of spinal cord interneurons are candidates to be members of the pattern generating circuits. The goals are 1) to continue the determination of the distribution of propriospinal neurons with short projections, 2) to determine the patterns of the axonal projections and terminations of the different groups of propriospinal neurons, and 3) begin to determine which groups of propriospinal neurons are, in fact, candidate members of the pattern generating circuits. We are determining the distribution of cells with short projections using retrograde transport of horseradish peroxidase following small iontophoretic injections into various regions of the spinal cord gray matter. Originally, we reacted the tissue sequentially using TMB and Ni-Co DAB as the chromogens to allow high quality Nissl staining. We are now switching our method to use glucose oxidase with Ni DAB as the chromogen. This technique is easier, has a high sensitivity, and permits a well differentiated stain. The complementary determination of the anterograde projections is being accomplished using iontophoretic injections of the anterogradely transported phaseolus lectin. The transported lectin is detected immunohisochemically. Neurons that are rhythmically active during fictive locomotion evoked by stimulation of the rhombencephalon are candidate members of the pattern generating circuits. We have modified our original plan to begin the survey of rhythmically active neurons using extracellular rather than intracellular recording to increase the number of neurons sampled per experiment. We will still attempt to activate the interneurons antidromically from the gray matter of the adjacent segments to identify the neurons as propriospinal. The position of the stimulating electrode will be based on the phaseolus data, and the locations of the recording sites will be compared with the locations of neurons determined by the HRP experiments. In subsequent years, we will extend this to intracellular recordings with intracellular labeling to determine the projection patterns and to examine descending and segmental inputs.